1. Field of the Invention
The present invention relates to a blower fan.
2. Description of the Related Art
Small and high-performance electronic devices, such as notebook PCs, produce a large amount of heat at CPUs and the like inside cases thereof. This makes it important to take measures against the heat. One common measure against the heat is to install blower fans inside the cases to discharge the heat. Meanwhile, there has been a demand for a reduction in thickness of the notebook PCs. Accordingly, the blower fans have been required to be reduced in thickness while reducing a deterioration in air-blowing performance.
Therefore, attempts have been made to reduce the height of blades of impellers to achieve a reduction in the thickness of the blower fans, and at the same time to increase the rotation rate of the blades to reduce the deterioration in the air-blowing performance. An increase in the rotation rate of the blades leads to increases in wind noise, noise caused by interference of a wind with a housing, and the like. Accordingly, it is important to reduce noise of such blower fans.
JP-A 2012-107577, for example, discloses a technique for an effective reduction in noise. According to this technique, a raised portion is provided in a surface of each of a plurality of blades, the surface facing rearward against a rotation direction of the blades, to prevent a vacuum from being generated in the vicinity of a surface of each blade, and furthermore, noises caused by winds propelled by the blades are caused to have a plurality of different types of frequencies and waveforms, whereby the effective reduction in noise is achieved.
Meanwhile, in a centrifugal fan disclosed in US 2012/0148394, a tongue portion is provided in an arc sidewall of a flowing channel in the vicinity of an air outlet defined in a side surface of a case. In a casing of a centrifugal fan disclosed in JP-A 2005-291049, a nose tongue of an outlet port includes an arc-shaped surface arranged to spread toward an end of the outlet port. In a multi-blade centrifugal fan disclosed in Japanese Patent No. 4183005, an inner surface of a scroll casing is in the shape of a logarithmic spiral given by Rs(θs)=r·exp((θs+β)·tan α), where θs is a central angle measured from a tongue portion of the scroll casing, Rs is the inside diameter of the scroll casing at the central angle (θs), r is the inside diameter of the scroll casing at the tongue portion, β is a correction value for the central angle (θs) of the scroll casing, and α is the enlargement ratio of the scroll casing.
In a blower fan whose housing is provided with the tongue portion, a noise is generated by interference of a wind sent from an impeller toward an air outlet with the tongue portion.